An optical system unit included in an image pickup device, such as a VCR-incorporated video camera, is mainly comprised of a plurality of lenses, an imaging element (CCD) and a spectroscope. The spectroscope comprises a spectral color filter or a spectral prism. In the optical system, optical components, such as iris, optical low-pass filter, and infrared-ray cut filter, are conventionally provided in an optical path of a photographic light or a peripheral area thereof in order to adjust an illumination and prevent the generation of a false color.
In an ordinary image taking in a visible-ray area by means of the image element comprising the CCD, an infrared-ray area constitutes an optical band interfering the image taking because the image taking including the infrared-ray area results in an image which cannot be recognized by viewers as a normal image. Therefore, as recited in No. 63-87889 of the Publication of the Unexamined Japanese Patent Applications, the infrared-ray cut filter is disposed in the optical path of the photographic light to thereby eliminate the infrared-ray area so that a favorable image taking of the visible-ray area can be realized. However, the image of the infrared-ray area is very effective in terms of image clarity in the case of photographing in a low-illumination environment such as photographing in nighttime and darkness.
As such, an image pickup device for realizing both of the conventional photographing and the photographing in the low-illumination environment is conventionally available. In the image pickup device, the infrared-ray cut filter is disposed in the optical path of the photographic light in the conventional photographing, the infrared-ray cut filter is retreated from the optical path of the photographic light when photographing in the low-illumination environment.
In realizing the foregoing image pickup device, it is necessary to provide a space in a peripheral area of the imaging element for disposing the infrared-ray cut filter and a mechanism for advancing/retreating the infrared-ray cut filter. Therefore, the foregoing constitution includes an optical system unit comprising a combination of the spectral color filter and imaging element (hereinafter, simply referred to as an optical system unit of a single plate type), and is employed in image pickup devices, which can afford the space in the peripheral area of the imaging element.
FIG. 1 shows an optical system (single plate-type optical system unit) of an image pickup device including the infrared-ray cut filter and its advancing/retreating mechanism. Referring to reference symbols in FIG. 1, 1a-1d denote a group of lenses, 2 denotes an imaging element, 3 denotes a spectral color filter, 4 denotes a dummy glass or the like, 5 denotes a low-pass filter, 6 denotes an infrared-ray cut filter, 7 denotes filters of different types, and 8 denotes an iris. Further, L1 denotes an optical path length from a front end of the group of lenses 1a-1d to the imaging element 2, Z denotes a central axis of an optical path of a photographic light, and P1 denotes a spatial length from a rear-most end of the group of lenses 1a-1d to the imaging element 2.
In the optical system (single plate-type optical system unit) shown in FIG. 1, the spatial length P1 can be relatively easily secured in a large dimension (approximately 10 mm or more) Therefore, the mechanism for advancing/retreating the infrared-ray filter 6 with respect to the optical path of the photographic light can be provided in the spatial length P1. The infrared-ray cut filter and its advancing/retreating mechanism are not shown in FIG. 1.
The realization of the constitution, however, is premised on that the peripheral area of the imaging element includes some spatial room for disposing the advancing/retreating mechanism for the infrared-ray cut filter 6. The premise can be satisfied in the before-mentioned single plate-type optical system unit, however, cannot be realized to a satisfactory level in the optical system unit comprised of a combination of the spectral prism and a plurality of imaging elements (hereinafter, referred to as an optical system unit of a spectral prism type).
FIG. 2 shows that the foregoing premise cannot be satisfied in the spectral prism-type optical system unit. Reference symbols shown in FIG. 2 refer to the same components as in FIG. 1, except for a reference numeral 9 denoting a spectral prism provided in place of the spectral color filter 3, L2 denoting an optical path length from the front end of the group of lenses 1a-1d to the imaging elements 2, R denoting an image-formation area of the optical system, and P2 denoting a spatial length from the rear-most end of the group of lenses 1a-1d to the spectral prism 9. The spatial length P2 is a part of a spatial length in the image-formation area R and houses therein the infrared-ray cut filter 6.
In the spectral prism-type optical system unit, it is necessary to provide the spectral prism 9 for spectrally separate an entering light into three primary colors in the image-formation area R of the optical system, and further to provide the imaging elements 2 for imaging the respective spectrally-separated three primary colors in the image-formation area R. Because of the requirement, the spatial length P2 in the spectral prism-type optical system unit is narrower than the spatial length P1 in the single plate-type optical system unit, as a result of which the advancing/retreating mechanism for the infrared-ray cut filter 6 cannot be possibly provided in the spectral prism-type optical system unit.
As a possible way to secure the space for disposing the advancing/retreating mechanism for the infrared-ray cut filter 6 in the spectral prism-type optical system unit is to extend the entire optical length L2. In that manner, the spatial length P2 can be enlarged when the image-formation area R is moved rearward in the optical path. However, the extension of the entire optical length L2 is followed by a relative reduction of a performance and product potency, which is, therefore, unrealizable in practical use.
The spectral prism-type optical system unit exerts a relatively good spectral performance and sensitivity. Because of that, it is desirable to dispose the infrared-ray cut filter and the mechanism for advancing/retreating thereof with respect to the optical path of the photographic light without extending the entire optical length L2 in the spectral prism-type optical system unit characterized as described.